1. Field of the Invention
The present invention relates to antenna devices capable of transmitting/receiving information and capable of receiving broadcasts such as telecasts, and to radio communication apparatuses including such antenna devices.
2. Description of the Related Art
Cellular phones are in widespread use as radio communication apparatuses for transmitting and receiving information. Recently, cellular phones capable of receiving, visualizing, and auralizing telecasts have been proposed. Antenna devices capable of both communication (telephone calls) and telecast reception may have a structure as shown in FIG. 10.
An antenna device shown in FIG. 10 includes an antenna 90 capable of both receiving signals in a frequency band for telecasts and transmitting/receiving signals in a frequency band for telephone calls (communication). The antenna 90 is mounted on a circuit board 100. A branching filter 80, a communication circuit 120, and a broadcast circuit 130 are provided on the circuit board 100. The signals in the frequency band for communication are transmitted between the antenna 90 and the communication circuit 120 by the branching filter 80. The signals in the frequency band for telecasts received by the antenna 90 are transmitted to the broadcast circuit 130.
Examples of such known antennas are disclosed in Japanese Unexamined Patent Application Publication No. 2002-9521 (Patent Document 1), Japanese Unexamined Patent Application Publication No. 2003-273620 (Patent Document 2), and Japanese Unexamined Patent Application Publication No. 2003-347815 (Patent Document 3).
When transmission and reception of the signals in the frequency band for communication and reception of the signals in the frequency band for telecasts are performed using the common antenna 90, signal waves in both frequency bands for communication and telecasts interfere with each other. Moreover, for example, the signals in the frequency band for communication in the communication circuit 120 enter the broadcast circuit 130 from the communication circuit 120 via the branching filter 80 through a path W shown in the FIG. 10. The entrance of the signals in the frequency band for communication presents problems of reduction in gain (sensitivity) of the signals in the frequency band for telecasts and degradation of antenna characteristics.
Moreover, antennas are required to be smaller as cellular phones become smaller. In general, the length of antennas (electrical length of antennas) can be set to an optimum value (a quarter of the signal wavelength) in either the frequency band for telecasts or the frequency band for communication. However, it is difficult to appropriately transmit and receive signals in a wide frequency range including the frequency band for telecasts and the frequency band for communication using only one antenna.
Specifically, when transmission and reception of signals in the frequency band for communication and reception of signals in the frequency band for telecasts are performed, the frequencies of the signals in the frequency band for telecasts ranging from 470 to 770 MHz, for example, are lower than those of the signals in the frequency band for communication (for example, frequency band of 800 MHz to 2 GHz), and moreover, the frequency band for telecasts is wider than that for communication. Therefore, in order to support both high and low frequency bands, antenna characteristics that support a sufficiently wide frequency range of 470 MHz to 2 GHz are required. However, it is difficult to properly transmit and receive signals of all channels in such a wide frequency range using only one antenna.
Furthermore, when the antenna is configured both to transmit and receive the signals in the frequency band for communication and to receive the signals in the frequency band for telecasts, the size of the antenna 90 increases, and the apparatus (antenna device) cannot be made compact.